1. Field of the Invention
The present invention relates generally to a device for heating to a desired infusion temperature a fluid to be supplied intravenously to a patient. More specifically, it relates to such a device that is (1) dimensioned to be wearable on the patient adjacent to the infusion situs (i.e., the location on the patient's body were the fluid is to be infused into the patient), and (2) configured to determine automatically when, and to provide warning if, the fluid flow rate in the device falls below a desired minimum threshold therefore, and/or gas (e.g., air) is present in the fluid. Although the present invention finds particular utility in heating of fluids which are to be supplied intravenously at relatively low flow rates (e.g., below about 2550 ml/hour for fluids to be heated to an infusion temperature of between 38-42 degrees C. from an input temperature into the device of 10 degrees C., or below about 3600 ml/hour for fluids to be heated to such an infusion temperature from an input temperature of 18 degrees C.), it should be understood that other utilities are also contemplated for the present invention (e.g., infusion of intravenous fluids at other temperatures and flow rates).
2. Brief Description of Related Prior Art
Many prior art techniques and devices exist for warming fluids to be infused intravenously into humans and other animals. One such conventional device is disclosed in U.S. Pat. No. 5,245,693 ("the '693 patent"). The '693 patent is directed to an intravenous fluid heater that includes a disposable cassette containing a heat exchanger. The preferred embodiment of the heat exchanger disclosed in the '693 patent includes a passageway-defining inner layer sandwiched between a pair of flexible, metal foil membranes. The inner layer defines an extended, e.g., serpentine, path for fluid to be warmed, and serves to space apart and insulate the metal foil membranes from one another. Inlet and outlet ports to the serpentine fluid path are defined in one of the two foil membranes. Heat generated by heating elements which sandwich the heat exchanger is transferred through the metal foil membranes to the fluid flowing through the serpentine path. The heating elements are designed to be graduated, that is, to generate more heat in the area of the inlet portion of the serpentine path than in the area of its outlet.
Unfortunately, the heating device disclosed in the '693 patent suffers from several disadvantages. First, since the heating elements of the '693 device apply non-uniform, graduated heating to the fluid traversing the flow path, the risk exists that, depending upon the fluid flow rate through the flow path, the fluid may become overheated (i.e., heated above the desired target temperature for infusion) at the portion of the heating elements applying elevated amounts of heat to the fluid. If sufficiently elevated, such overheating can damage certain types of intravenously delivered fluids (e.g., blood and blood-products, if the overheating is above about 41 degrees C.).
Also, the heating device disclosed in the '693 patent is not wearable by the patient adjacent the fluid infusion situs. This means that the length of tubing required to deliver the heated fluid from the device to the infusion situs may vary depending upon where the device is positioned relative to the patient, but will always be longer than that which would be required if the device were being worn by the patient at or near the infusion situs. This means that in the infusion arrangement disclosed in the '693 patent, the temperature of the heated fluid exiting the heating device will always drop, prior to being infused into the patient, more than would be the case if the heating device were wearable adjacent the infusion situs. The temperature drop of the heated fluid can be especially pronounced at the aforesaid relatively low fluid flow rates. Unfortunately, a significant proportion of intravenous fluid infusions take place at such low flow rates.
A yet further disadvantage of the '693 patent's heating arrangement is that although means are included for reducing gas bubble formation in the infusion fluid, no means are provided for automatically determining whether such bubbles are present in the fluid or whether there has been reduction in fluid flow rate, and for taking appropriate action (e.g., providing warning and/or stopping fluid flow into the patient) in the event such conditions are determined to be present. As will be appreciated by those skilled in the art, if left unchecked these types of conditions can be, at minimum, deleterious to patient well-being, and at most, life-threatening.
An additional disadvantage of the heating device disclosed in the '639 patent results from its use of metal foil membranes. As was acknowledged by the patentee of the '639 patent during prosecution of said patent, if such metal foil membranes become distorted in use, the fluid being heated can become "spot heated" to elevated temperatures at the locations of such distortions. Such spot heating can result in damage to the fluid being heated.
Another conventional infusion fluid warming device is disclosed in U.S. Pat. No. 5,254,094 ("the '094 patent"). In the arrangement disclosed in the '094 patent, a box which may be attached to a patient's arm is provided. Two chambers are included in the box, containing a heat exchanger element constructed from a continuous length of stainless steel tubing in the form of two parallel coils which are connected to each other by a straight length of tubing. The box includes a passage between the chambers such that a warming fluid may be introduced through an aperture in the box into one of the chambers, flow into the other chamber, and then exit the warmer through another aperture in the box. The infusion fluid to be warmed is supplied to the coils through a first flexible plastic inlet tube and discharged for infusion into a patient through a second flexible plastic tube. The warming fluid is supplied via fluid supply tubing to the box from a separate fluid source that is not dimensioned or suitable for being worn by the patient, such as a water heater. A temperature sensor located in the infusion fluid path between the box and the infusion situs may be provided for generating signals indicative of the temperature of the infusion fluid for provision to a microprocessor contained in the same unit comprising the water heater. The microprocessor also receives outputs from a water temperature sensor and controls the water heater, based upon the outputs from these sensors and a desired infusion fluid temperature set by the user, so as to maintain the heating water at a temperature for heating the infusion fluid to the desired temperature.
Disadvantageously, use of a warming fluid/infusion fluid type of heat exchanger, and a warming fluid heater that is remote from the heat exchanger and not wearable by the patient, make '094 patent's arrangement bulky, and relatively difficult to move and set up for use. Also disadvantageously, if even a single crack, pin-hole, imperfect seal, or other opening exists in the infusion fluid tubing/fittings in the heat exchanger, the infusion fluid may become contaminated with the warming fluid. Additionally, as is the case in the '693 patent, the '094 patent discloses no means for automatically determining whether gas bubbles are present in the infusion fluid or there has been reduction in infusion fluid flow rate, and for taking appropriate action (e.g., providing warning and/or stopping fluid flow into the patient) in event such conditions are determined to be present.
Other examples of infusion fluid warming prior art are disclosed in U.S. Pat. Nos. 5,381,510, 4,731,072, 3,443,060, 3,475,590, 3,485,245, 3,590,215, 3,614,385, 3,640,283, 3,853,479, 4,038,519, 4,108,146, 4,167,663, 4,293,762, 4,309,592, 4,938,279, 4,847,470, 4,574,876, 3,399,536, 4,962,761, 5,125,069, 4,908,014, 4,906,816, 4,844,074, 4,707,587, 4,759,749, 4,782,212, 4,801,777, 4,680,445, 4,678,460, 4,532,414, 4,464,563, 4,314,143, 4,356,383, and 4,878,537. Unfortunately, the prior art disclosed in each of these patents suffers from the aforesaid and/or other disadvantages and drawbacks.